Conventionally, stereophonic sound systems using a right and a left speakers or the like to virtually positioning a stereo sound for a listener are known and manufactured. For example, Japanese patent documents JP3657120 and JP3880236 (equivalent to U.S. Pat. Nos. 6,763,115 and 6,842,524) disclose such technique.
However, by conducting an experiment, the inventor of the present disclosure found that, in those techniques, sound positioning effects by using two speakers installed on the right and the left side of listener (a test subject, or testee) illustrated in FIGS. 10A and 10B are not sufficient in terms of positioning a virtual sound source, especially in a front field of the listener.